I had exactly the same problem but I am using the latest ActiveSync 3.8 so I cant see it is a version problem.
I did two different things so I am not sure which sorted it

but I have got mine working fine now.
1. I removed the IP settings that I had put on the device and allowed it to supply the ip address automatically (even though all my other networking on the device was fine)
2. I changed the devices name from the default "Pocket_pc" to something more helpful, I chose "mda3". This is in the Start > Settings > System > About > Device ID menu just incase any one wants to know.
Then finally I synced it to the PC via the cradle and then, once synced, removed it from the cradle and synced it via wifi (checking that the setting in activesync was ticked for wireless connections)
I know mikey has probably long forgotten or resolved this but I thought I would post this just in case anyone else is having the same problem.
